1. Field of the Invention
The present invention relates to a cage to be interposed between a pair of vertebral bodies in place of a degenerative intervertebral disk after it is removed, and, more particularly to a cage with an excellent home position maintaining ability which can be placed between a pair of vertebral bodies easily, and in which after the cage is placed, its home position is maintained without moving the cage.
2. Description of the Related Art
To place a cage between a pair of vertebral bodies in place of a degenerative intervertebral disk after it is removed has been conventionally carried out, and cages of various shapes have been developed. Japanese Patent Application Laid-open No. H8-266565 describes a cage of a shape closest to that of the present invention.
A cage 81 described in Japanese Patent Application Laid-open No. H8-266565 has a structure as shown in FIG. 1. That is, a cage body 83 of the cage 81 is tapered as an entire configuration, and left and right side surfaces 85 of the cage body 83 are formed in a smooth flat surface. Upper and lower surfaces of the cage body 83 are formed in an arc curved surface. A longitudinal hole 87 is formed in the cage body 83, a cross-sectional shape of the hole 87 is of a vertically long circular shape, and a rear end of the hole 87 is closed with a wall portion 89. The left and right side surfaces 85 of the cage body 83 are respectively provided with communication holes 91A and 91B which are in communication with the hole 87, and long holes 93, which are long in the longitudinal direction, are formed in upper and lower surfaces of the cage body 83.
A thread 95 is formed on an outer peripheral surface of the cage body 83, more precisely, upper and lower surfaces of the cage body 83. Although the thread 95 is cut by the fact that the side surfaces 85 are formed in a flat surface and the upper and lower surfaces of the cage body 83 are formed with the long holes 93, one thread 95A of the rear end makes the round of the cage body 83 without being cut. Tool engaging portions 97 with which a tool used for setting the cage 81 between a pair of vertebral bodies is engaged are formed in front end surface of the cage body 83.
Therefore, in the cage 81, to place the cage 81 between a pair of upper and lower vertebral bodies after a degenerative intervertebral disk is removed, it is necessary to push the cage body 83 in between the pair of vertebral bodies while rotating the cage body 83 so that the thread 95A formed in the rear end of the cage body 83 is first engaged with the upper and lower vertebral bodies. Thus, a placing operation of the cage body is rather troublesome.
Further, there is a problem that the producing cost of the cage becomes high due to the following reasons, i.e., the cage body 83 is tapered, a cross-sectional shape of the hole 87 is long in the vertical direction, shapes and sizes of the communication holes 91A and 91B and the long hole 93 are different from each other.
Further, although the thread 95 is cut, the thread 95 has a long linear shape. Thus, the thread 95 has a problem in engaging ability with the vertebral body, and there is also a problem that the cage 81 is occasionally displaced in position after the cage 81 is placed between a pair of vertebral bodies.